1. Field of the Invention
The present invention relates generally to an actively controlled suspension system for an automotive vehicle, which achieves suspension control depending upon vehicle driving condition for suppressing attitude change. More specifically, the invention relates to an actively controlled suspension system which may perform anti-roll suspension control with high precision irrespective of affecting factors, such as delay in phase caused in a control system, load condition on the vehicle or so forth.
2. Description of the Background Art
European Patent First Publication No. 0 283 004 discloses an actively controlled suspension system including an anti-roll control roop which is responsive to lateral acceleration to be exerted on a vehicle to adjust suspension characteristics. The control loop employed suspension system is variable of response characteristics to vehicular rolling depending upon the vehicle speed for varying anti-rolling suspension control characteristics. In a range of the vehicle speed in which the greater loop gain of the control loop, causes significant and unacceptable level of self-induced lateral vibration, the gain is varied to be the smaller value than that in other vehicle speed range. Namely, since the greater gain may affect for causing self-induced lateral vibration in substantially low vehicle range, the gain main be adjusted to smaller value while vehicle speed is lower than a given value. Also, European Patent First Publication No. 0 285 153 discloses an actively controlled automotive suspension system employing a factor representative of self-induced attitude change creative factor as correction factor in attitude regulating suspension control. Suspension characteristics is controlled on the basis of rolling and/or pitching energy representing parameter. The correction factor representative of self-induced attitude change is derived on the basis of the vehicular attitude change velocity for canceling the self-induced rolling and/or pitching. By canceling the self-induced rolling and/or pitching creative component from the rolling and/or pitching acceleration factor, higher rolling and/or pitching response characteristics can be provided so as to satisfactorily high dricing stability and riding comfort.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-295714, which is assigned to the common assignee to the present invention, discloses one typical example of prior proposed actively controlled suspension system for performing anti-rolling and/or anti-pitching suspension controls. The disclosed system employs a lateral acceleration sensor and a longitudinal acceleration sensor for monitoring vehicular attitude change magnitude indicative parameters for performing attitude change suppressive control.
As will be appreciated, the lateral acceleration sensor employed in the actively controlled suspension system monitors lateral force exerted on the vehicular body and whereby detects possible magnitude of vehicular rolling. The system controls suspension characteristics of respective suspension systems suspending vehicular body on respective front-left, front-right, rear-left and rear-right wheels. In the foregoing publication, a singular lateral acceleration sensor for providing common anti-rolling control parameter for controlling suspension characteristics of respective suspension systems.
When single lateral acceleration sensor is to be mounted on the vehicle body, it is preferred to locate the sensor at or nearby the gravity center. When the lateral acceleration sensor is provided at the gravity center, lag factors, such as mechanical lag in control valve, phase delay in control system, will substantially affect for response characteristics of anti-rolling control.
In order to improve this, it has been proposed to locate the lateral acceleration sensor at an orientation frontwardly shifted from the gravity center. In such case, a difficulty is encountered in that, since the front portion of the vehicle body is generally occupied by an automotive engine and a power transmission, an appropriate position to install the sensor is difficult to find out. Furthermore, for avoiding influence of rolling motion of the vehicular body in monitoring the lateral acceleration, the height level to install the lateral acceleration sensor has to be as close as possible to vehicular body rolling axis. This further makes it difficult to find out appropriate position to install the sensor.
In addition, as will be appreciated, the gravity center of the vehicular body shifts from an initial position depending upon load condition, such as increasing or decreasing of luggages or passengers, anti-rolling control characteristics tends to be varied in relation to position of the gravity center. This makes the vehicle driving characteristics unstable due to variation of anti-rolling characteristics.